Prostaglandin I.sub.2 (PGl.sub.2)is a physiologically active natural substance having the following structural formula, which is biosynthesized from Prostaglandin H.sub.2 (PGH.sub.2)in the metabolic process in vivo called arachidonate cascade. ##STR3## (see Nature, 263, 663(1976), Prostaglandins, 12, 685(1976), ibid, 12, 915(1976), ibid, 13, 375 (1977) and Chemical and Engineering News, Dec. 20, 17(1976)).
PGl.sub.2 has been confirmed to possess not only a very strong inhibitory activity on blood platelet aggregation but an inhibitory activity on blood platelet adhesion, a vasodilating activity, an inhibitory activity on gastric acid secretion etc. Therefore, it has been considered that PGl.sub.2 is useful for the prevention and/or the treatment for thrombosis, arteriosclerosis, ischemic heart diseases, gastric ulcer, hypertension etc. But its application for pharmaceuticals is limited because of its chemical instability and difficulty of separation of the actions according to purpose. Accordingly, various PGl.sub.2 derivatives have been synthesized and much research has been carried out for the maintenance and the separation of the actions. However, we have not had satisfactory results yet.
Recently, in order to solve two problems above described, the research for PGl.sub.2 receptor agonists which don't have PG skeleton has been carried out.